One of the primary objectives being sought today by engineers in the packaging field is increased speed of operation. One of the critical factors in being able to successfully increase speed in a packaging line for paperboard cartons is the sealing of the carton in an efficient manner. Of course, efficiency presupposes that the carton can be sealed liquid tight so as to prevent leakage during use. One of the primary users of paperboard cartons is the food industry which makes it mandatory that sealing is accomplished taking into account both objectives; that is, high speed and liquid tightness.
Within the food industry, one of the most popular paperboard cartons today is the coated paperboard tray with a lid, either provided separately or as a part of an integral hinged cover. These trays are particularly adapted for packaging convenience food that can be prepared in the microwave oven. The most popular form of this carton or tray is shown in the present applicant's own U.S. Pat. No. 4,304,352 to Humphries, issued Dec. 8, 1981. As can be seen by reviewing this patent, the side panels and the lid are provided with mating flanges that are sealed together to form the liquid tight carton suitable for many products, but particularly suited for frozen food entrees or the like.
In the prior art, it is common practice to provide adhesive between the flanges, such as heat activated thermosetting adhesive and/or sprayed high contact adhesive, such as illustrated and claimed in applicant's prior U.S. Pat. No. 4,249,978 to Baker, issued Feb. 10, 1981. Once the adhesive is applied/activated, the carton is fed along a continuous operating conveyor with side positioned pressure members engaging the flanges for sealing. Since the flanges in the tray-type carton shown in the U.S. Pat. No. 4,304,352 are continuous along the sides of the carton and the side panels are angled outwardly, there is generally no problem in engaging the flanges to apply the pressure for the sealing process. A standard approach is to use two pairs of rollers, one pair on each side, for sealing the flanges on two sides; and then rotating the carton and sealing the other two sides by passing through two more pairs of rollers. A common approach to this manner of sealing is shown in the U.S. Pat. No. 4,969,306 to Wallin, issued Nov. 13, 1990.
However, in any situation where there is a complication in the shape of the carton or lid, or where special consideration must be given to some other operating factor, the only known successful approach, insofar as I am aware, is to feed the tray-like carton intermittently along the feed path and then apply the lid at one stop station and seal the lid at a downstream stop station. A typical approach with respect to this type of special packaging machine is shown in the Oxborrow U.S. Pat. No. 4,626,234, issued Dec. 2, 1986. As will be readily recognized, the objective of increased speed of operation must be completely abandoned when this approach is taken.
Recently, an innovation in carton design has led to the introduction of a special tray-type carton that requires special consideration for sealing. The particular carton innovation provides at least one of the flanges of the side panel or cover of the carton as an inverted U-shaped flange extending outwardly from the side. Also, the side panels of the carton extend substantially perpendicular rather than slanted outwardly, as is more common, such as shown in the Humphries U.S. Pat. No. 4,304,352.
As illustrated in the U.S. Pat. No. 4,626,234, the first and apparently only approach suggested by the prior art, is to seal the carton flanges of the side panels and cover by intermittent feed with a reciprocating platen at a stop station. It can be readily recognized that the use of rotating presser rollers along the sides of the conveyor does not work due to the inevitable interference provided by the depending portions of the U-shaped flange. Thus, prior to the present invention, engineers were stuck with using the intermittent feed/reciprocating platen sealing arrangement.
As recognized by those skilled in the art, the carton construction with the inverted U-shaped flanges provides a significant strengthening of the carton, as well as providing convenient side handles.
Thus, it would be very desirable to be able to take advantage of the new carton if the benefits of continuous operation along the conveyor feed path can also be obtained.